MIL-P-288B
to within l/32-inch of the bottom. A 3/64-inch diameter hole shall be drilled
through the bottom center of the post. A plastic washer (see 3.3.2) of 1-1/2-
inch diameter shall be provided over the two movable arms. A knurled ring nut
shall be provided to secure the washer and movable arms, yet allow for free
axial movement of the arms.
3.6.6 Clamps. Each movable arm shall be provided with a "U"-shaped brass
clamp. The bottom of each clamp shall have a threaded post secured so that it
will not turn. The top of the clamps shall be shaped at the inner edge, so
that they will not obstruct the view of any of the numerals or graduations on
the vernier. A black molded plastic knob, with a brass thread insert, shall
be used for tightening the clamp. A brass washer, 1/32-inch thick, shall be
provided between the knob and the clamp.
3.7 Construction, type II, class 2 protractor.
3.7.1 Assembly. The assembled protractor shall consist of a body having
a graduated semi-circle with a movable arm containing a vernier scale and
thumb nut clamp (see figure 3).
3.7.1.1 Accuracy. The total error of the assembled protractor shall not
exceed 6 minutes of arc. The body, arm, and center post shall be assembled so
that the beveled edge of the arm coincides radially at all angles with the
graduations on the edge of the body. The graduated edge of the arm shall
bisect the grid squares diagonally when alined with the 45- and 315-degree
graduations, and shall coincide vertically with the line from the radial center
to the zero graduation and horizontally with the line through the 90- and 270-
degree graduations. The clamping arrangement shall allow the arm to be rotated
without friction on the top surface of the body, yet allow for positive locking
of the arm at any desired position.
3.7.2 Body. The body shall be plastic (see 3.3.2). The body shall be a
semicircular disk, 4.875 ± 0.010 inches high, 4.000 ± 0.005 inches in radius,
and 0.080 ± 0.010 inch thick. The center of radius shall be located 7/8-inch
above the lower edge of the body on the vertical centerline. A clearance hole
and keyway for the center post and a counterbored hole on the underside of the
body shall be provided for the center post flange, at the center of radius of
the body.
3.7.2.1 Markings. All markings shall be on the underside and protected
with a transparent plastic cover sheet from 0.003- to 0.005-inch thick of a
material compatible with the body.
3.7.2.2 Graduations. The graduations shall be either stamped or engraved
with a tolerance of plus or minus 5 minutes of arc. The circular edge shall
be graduated in half degrees with lines 0.004- to 0.010-inch wide. The length
of the graduation marks shall be 1/2 degree marks, 1/8-inch; 1 degree marks,
3/16-inch; 5 and 10 degree marks, 5/16-inch. Each 10-degree line shall be
designated with numerals 5/64-inch high. The outer row shall be numbered to
read from left to right, from 260 to 350 degrees with zero at the top, and
continuing up to 100 degrees, The second or inner row shall read from left to
right beginning with 80 and continuing to 280 degrees. All markings shall be
filled or printed with permanent waterproof ink or enamel.
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